Aerodynamics of Wind Turbines

Part -3

Airfoils and general aerodynamic concepts

Wind turbine blades use airfoil sections to develop mechanical power.

The width and length of the blades are a function of the desired aerodynamic performance and the maximum desired rotor power (as well as strength considerations).

Before examining the details of wind turbine power production, some airfoil aerodynamic principles are reviewed here.

Basic airfoil terminology

Camber = distance between mean camber line (mid-point of airfoil) and the chord line (straight line from leading edge to trailing edge)Thickness = distance between upper and lower surfaces (measured perpendicular to chord line)Span = length of airfoil normal to the cross-section

Camber

Thickness

Examples of standard airfoil shapesNational Advisory Committee for Aeronautics

NACA 0012 = 12% thick symmetric airfoilNACA 63(2)-215 = 15% thick airfoil with slight camberLS(1)-0417 = 17% thick airfoil with larger camber

Lift, drag and non-dimensional parameters

Airflow over an airfoil produces a distribution of forces over the airfoil surface.

The flow velocity over airfoils increases over the convex surface resulting in lower average pressure on the 'suction' side of the airfoil compared with the concave or 'pressure' side of the airfoil.

Meanwhile, viscous friction between the air and the airfoil surface slows the airflow to some extent next to the surface.

Lift force - defined to be perpendicular to direction of the oncoming airflow. The liftforce is a consequence of the unequal pressure on the upper and lower airfoil surfaces

Drag force - defined to be parallel to the direction of oncoming airflow. The drag force is due both to viscous friction forces at the surface of the airfoil and to unequal pressure on the airfoil surfaces facing toward and away from the oncoming flow

Pitching moment - defined to be about an axis perpendicular to the airfoil cross-section

The resultant of all of these pressure and friction forces is usually resolved into two forces and a moment that act along the chord at c / 4 from the leading edge (at the 'quarter chord').

These forces are a function of Reynolds numberRe = U L / (L is a characteristic length, e.g. c)

Velocity = U

The 2-D airfoil section lift, drag and pitching moment coefficients are normally defined as:

A = projected airfoil area = chord x span = c l

Other dimensionless parameters that are important for analysis and design of wind turbines include the power and thrust coefficients and tip speed ratio, mentioned earlier and also the pressure coefficient:

and blade surface roughness ratio:

Airfoil aerodynamic behaviourThe theoretical lift coefficient for a flat plate is:

which is also a good approximation for real, thin airfoils, but only for small .

Lift and drag coefficients for a NACA 0012 airfoil as a function of and Re

Airfoils for HAWT are often designed to be used at low angles of attack, where lift coefficients are fairly high and drag coefficients are fairly low.

The lift coefficient of this symmetric airfoil is about zero at an angle of attack of zero and increases to over 1.0 before decreasing at higher angles of attack.

The drag coefficient is usually much lower than the lift coefficient at low angles of attack. It increases at higher angles of attack.

Note the significant differences in airfoil behaviour at different Re. Rotor designers must make sure that appropriate Re data are available for analysis.

Lift at low can be increased and drag reduced by using a cambered airfoil such as this DU-93-W-210 airfoil used in some European wind turbines:

Note non-zero lift coefficient at zero incidence.

Data shown: Re = 3 x106

Lift coefficient

Drag and momentcoefficients

Another wind turbine airfoil profile: Lift and drag coefficients for a S809 airfoil at Re = 7.5x107

Attached flow regime:At low (up to about 7o for DU-93-W-210), flow is attached to upper surface of the airfoil. In this regime, lift increases with and drag is relatively low.

High lift/stall development regime:Here (from about 7-11o for DU-93-W-210), lift coeff peaks as airfoil becomes increasingly stalled. Stall occurs when exceeds a critical value (10-16o, depending on Re) and separation of the boundary layer on the upper surface occurs. This causes a wake above the airfoil, reducing lift and increasing drag. This can occur at certain blade locations or conditions of wind turbine operation. It is sometimes used to limit wind turbine power in high winds. For example, many designs using fixed pitch blades rely on power regulation control via aerodynamic stall of the blades. That is, as wind speed increases, stall progresses outboard along the span of the blade (toward the tip) causing decreased lift and increased drag. In a well designed, stall regulated machine, this results in nearly constant power output as wind speeds increase.

Flat plate/fully stalled regime:

In the flat plate/fully stalled regime, at larger up to 90o, the airfoil acts increasingly like a simple flat plate with approximately equal lift and drag coefficients at of 45o and zero lift at 90o.

Illustration of airfoil stall

Airfoils for wind turbines

Typical blade chord Re range is 5 x 105 – 1 x 107

1970s and 1980s – designers thought airfoil performance was less important than optimising blade twist and taper.

Hence, helicopter blade sections, such as NACA 44xx and NACA 230xx, were popular as it was viewed as a similar application (high max. lift, low pitching moment, low min. drag).

But the following shortcomings have led to more attention on improved airfoil design:

Operational experience showed shortcomings (e.g. stall controlled HAWT produced too much power in high winds, causing generator damage).

Turbines were operating with some part of the blade in deep stall for more than 50% of the lifetime of the machine.

Peak power and peak blade loads were occurring while turbine was operating with most of the blade stalled and predicted loads were 50 – 70% of the measured loads!

Leading edge roughness affected rotor performance. Insects and dirt output dropped by up to 40% of clean value!

Momentum theory and Blade Element theory

The actuator disk approach yields the pressure change across the disk that is, in practice, produced by blades.

This, and the axial and angular induction factors that are a function of rotor power extraction and thrust, will now be used to define the flow at the airfoils.

The rotor geometry and its associated lift and drag characteristics can then be used to determine- rotor shape if some performance parameters are known, or- rotor performance if the blade shape has been defined.

Analysis uses:

Momentum theory - CV analysis of the forces at the blade based on the conservation oflinear and angular momentum.

Blade element theory – analysis of forces at asection of the blade, as a function of blade geometry.

Results combined into “strip theory” or blade element momentum (BEM) theory.

This relates blade shape to the rotor's ability to extract power from the wind.

Analysis encompasses:

- Momentum and blade element theory.

- The simplest 'optimum' blade design with an infinite number of blades and no wake rotation.

- Performance characteristics (forces, rotor airflow characteristics, power coefficient) for a general blade design of known chord and twist distribution, including wake rotation, drag, and losses due to a finite number of blades.

- A simple 'optimum' blade design including wake rotation and an infinite number of blades. This blade design can be used as the start for a general blade design analysis.

Momentum theory

We use the annular control volume, as before, with induction factors (a, a’ ) being a function of radius r.

Similarly, from conservation of angular momentum, the differential torque, Q, imparted to the blades (and equally, but oppositely, to the air) is:

Together, these define thrust and torque on an annular section of the rotor as functions of axial and angular induction factors that represent the flow conditions.

Applying linear momentum conservation to the CV of radius r and thickness dr gives the thrust contribution as:

Rotor Design(Blade element theory)

The forces on the blades of a wind turbine can also be expressed as a function of Cl , Cd and .

For this analysis, the blade is assumed to be divided into N sections (or elements). Assumptions:- There is no aerodynamic interaction between elements.- The forces on the blades are determined solely by the lift and drag characteristics of the airfoil shape of the blades.

Diagram of blade elementsc = airfoil chord length; dr = radial length of element

r = radius; R = rotor radius; = rotor angular velocity

Overall geometry for a downwind HAWT

analysis; U = velocity of undisturbed flow; = angular velocity

of rotor; a = axial induction factor

Note:

Lift and drag forces are perpendicular and parallel, respectively, to an effective, or relative, wind.

The relative wind is the vector sum of the wind velocity at the rotor, U (1 - a), and the wind velocity due to rotation of the blade.

This rotational component is the vector sum of the blade section velocity, r, and the induced angular velocity at the blades from conservation of angularmomentum, r / 2, or:

Rotor Design(Blade element theory)

The air hits the blade in an angle α which is called the “angle of attack”. The reference line” for the angle on the blade is most often “the chord line” .

The force on the blade F can be divided into two components – the lift force F L and the drag force F D and the lift force is – per definition – perpendicular to the wind direction.

Rotor Design(Blade element theory)

The lift force is given as

And the drag force

The ratio GR=CL / CD is called the “glide ratio”,

Normally we are interested in at high glide ratio for wind turbines as well as for air planes.

Rotor Design(Blade element theory)

Pitch angle, β, and chord length, c, after Betz

To design the rotor we have to define the pitch angle β and the chord length c. Both of them depend on the given radius, that we are looking at therefore we sometimes write β(r) and c(r).

Angles, that all depends on the given radius

• γ(r) = angle of relative wind to rotor axis• φ(r) = angle of relative wind to rotor plane• β(r) = pitch angle of the blade

The blade is moving up wards, thus the wind speed, seen from the blade, ismoving down wards with a speed of u.

Rotor Design(Blade element theory)

Pitch angle, β, and chord length, c, after Betz

Betz does not include rotation of the wind (No Wakes). Therefore

Here ω is the angular speed of the rotor

RU

=

R(1-a)U

(r) =tan-1

For a=1/3 3r2R

(r) =tan-1

(r) = +

and2R3r

(r) =tan-1

The pitch angle (r) =tan-1 -2R3r

ru

Rotor Design

Pitch angle, β, and chord length, c, after Betz

Most often the angle is chosen to be close to the angle, that gives maximum glide ration, that means in the range from 5 to 10°, but near the tip of the blade the angle is sometimes reduced.

Chord length, c(r):For one blade element in the distance r from the rotor axis with the thickness dr the lift force is

and the drag force

For the rotor plane (torque) we have

X

DLDL

Crdr)r(c

r)cosCsinC(dr)r(cr)cosdFsindF(dQ

2

2

2

2

Rotor Design

the thrust

sinrCdr)r(cdQ L

2

2

Now, in the design situation, we have C L >> CD

For N blades sinrCdr)r(cNdP L

2

2

According to Betz, the blade element would also give

)rdr(U

NAU)a(adP 2227

1612

332

9

4

1

9

162

3

22

R

rC

R

N)r(c

sinru,coswU

D,L

and

Using

Note: effect of drag is to decrease torque and, hence, power, but to increase the thrust loading.

Thus, blade element theory gives 2 equations: normal force (thrust) and tangential force (torque), on the annular rotor section as a function of the flow angles at the blades and airfoil characteristics.

These equations will be used to get blade shapes for optimum performance and to find rotor performance for an arbitrary shape.

Rotor Design

These equations may be used to find the chord and twist distribution of the Betz optimum blade.

Example: Given = 7, R = 5m, CL= 1, CD /CL is minimum at = 7, and there are 3 blades (N = 3) we can use: and

together with to obtain the changes in chord, twist angle (= 0 at tip), angle of relative wind, and section pitch, with radial distance, r/R, along the blade:

2R3r

(r) =tan-19

4

R

rλλ

1

C

πR

9N

16c(r)

22DL,

(r) = +

Twist and chord distribution for a Betz optimum blade(r/ R = fraction of rotor radius)

Hence, blades with optimized power production have increasingly larger chord and twist angle on approaching the blade root (r0). Actual shape depends on difficulty/cost of manufacturing it.

Blade chord for example Betz optimum blade

met

res

Blade twist angle for example Betz optimum blade